1. Field of the Invention
The present invention relates to a communication apparatus, and more particularly to a wireless communication apparatus.
2. Related Background Art
A spectrum diffusion (spread spectrum) communication system has been known as one of a plurality of communication systems. In the spectrum diffusion communication system, a transmitting station converts a base band signal of source data to be transmitted, such as digital audio/video data, to a base band signal having a much wider band width than that of the source data, by using a diffusion (spread) code such as a pseudo noise code. The converted signal is modulated by PSK or FSK modulation system into a high frequency signal, which is then transmitted.
A receiving station demodulates the source data by reverse-diffusion (reverse-spread) which correlates with the received signal by using the same diffusion code as that used in the transmitting station.
An outline of the spectrum diffusion communication is explained with reference to FIGS. 1 to 3.
A source data d(t) applied to a multiplier 6-1 in the transmitting station has a waveform shown by .circle. in FIG. 2, and a spectrum shown by .circle. in FIG. 3.
A diffusion code P(t) which is another input signal to the multiplier 6-1 of FIG. 1 has a waveform shown by .circle. in FIG. 2 and a spectrum shown by .circle. in FIG. 3. Since the diffusion code P(t) varies much more violently than the source data d(t), the spectrum .circle. in FIG. 3 has a much wider band than the spectrum .circle. .
The data d(t) is multiplied by the diffusion code P(t) by the multiplier 6-1 of FIG. 1 so that it is diffused (spread). The output of the multiplier 6-1 has a waveform shown by .circle. in FIG. 2 and a spectrum shown by .circle. in FIG. 3 which has a substantially equal band width to that of the spectrum .circle. . The output signal of the multiplier 6-1 is mixed by a mixer 6-2 with a carrier generated by a local oscillator 6-3. A waveform of the carrier is shown by .circle. in FIG. 3. The output of the mixer 6-2 has a waveform shown by .circle. in FIG. 2 and a spectrum shown by .circle. in FIG. 3, and it is sent via an antenna 6-3.
In the receiving station, the spectrum of the signal received via an antenna 6-5 of FIG. 6 usually includes the desired signal sent from the transmitting station as well as various signals as shown by .circle. in FIG. 3. They include noises, signals sent by other stations and a narrow band interference signal. The received signal including those signals is mixed by a mixer 6-6 with the same diffusion code P(t) as that used in the transmitting station, and it is converted to a signal having a spectrum shown by .circle. in FIG. 8. Namely, the desired signal in the received signal which corresponds to the diffusion code P(t) is reverse-diffused (reverse-spread) and the band width thereof is narrowed in accordance with the band width of the source data. On the other hand, undesired signal, that is, the signal from other station having no identical diffusion code or narrow band interference signal is diffused and converted to a wide band signal.
The diffusion code used for modulation and demodulation is set such that mutual correlation in each communication channel is sufficiently small. Accordingly, when a signal diffused (spread) by a different code is demodulated by reverse-diffusion, it becomes a wide band noise. As a result, only the desired signal can be extracted. Accordingly, multi-connection by code division is attained. Namely, a common frequency may be shared by a plurality of communication channels.
The output signal of the mixer 6-6 is applied to a band-pass filter 6-7 having a band width corresponding to the band width of the source data so that it is converted to a narrow band signal having a spectrum shown by .circle. in FIG. 3. The narrow band signal is demodulated such as by PSK demodulation by a demodulator 6-8 of FIG. 1 to reproduce the source data.
When a number of moving (mobile) radio stations conduct multi-connection by the code division to a multi-connection apparatus which is a single fixed station, the following problem arises. An electric field strength received by the fixed station is higher as the signal is sent from a nearer moving (mobile) station to the fixed station. Accoridngly, when the signal sent from a distant moving station is demodulated, the signal sent from a nearby moving station functions as a wide band noise but an electric field strength thereof is much higher than that of the signal from the distant moving station and a noise power which is supplied to an intermediate frequency filter 5 is unnegligibly large. Thus, in the prior art, the performance of the multi-connection is significantly deteriorated by the distance between the radio stations.